This invention is in the field of plant and fungal molecular biology. More specifically, this invention pertains to nucleic acid fragments encoding proteins involved in the riboflavin biosynthetic pathway of plants or fungi.
Riboflavin, vitamin B2, is the precursor of flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD), essential cofactors for a number of mainstream metabolic enzymes that mediate hydride, oxygen, and electron transfer reactions. Riboflavin-dependent enzymes include succinate dehydrogenase, NADH dehydrogenase, ferredoxin-NADP+ oxidoreductase, acyl-CoA dehydrogenase, and the pyruvate dehydrogenase complex. Consequently, fatty acid oxidation, the TCA cycle, mitochondrial electron-transport, photosynthesis, and numerous other cellular processes are critically dependent on either FMN or FAD as prosthetic groups. Other notable flavoproteins include glutathione reductase, glycolate oxidase, P450 oxido-reductase, squalene epoxidase, dihydroorotate dehydrogenase, and xcex1-glycerophosphate dehydrogenase. Genetic disruption of riboflavin biosynthesis in E. coli (Richter et al., J. Bacteriol. 174:4050-4056 (1992)) and S. cerevisiae (Santos et al., J. Biol. Chem. 270:437-444 (1995)) results in a lethal phenotype that is only overcome by riboflavin supplementation. This is not surprising, considering the ensemble of deleterious pleiotropic effects that would occur with riboflavin deprivation.
Riboflavin is synthesized by plants and numerous microorganisms, including bacteria and fungi (Bacher, A., Chemistry and Biochemistry of Flavoproteins (Mxc3xcller, F., ed.) vol. 1, pp. 215-259, Chemical Rubber Co., Boca Raton, Fla. (1990)). Since birds, mammals, and other higher organisms are unable to synthesize the vitamin and, instead, rely on its dietary ingestion to meet their metabolic needs, the enzymes that are responsible for riboflavin biosynthesis are potential targets for future antibiotics, fungicides, and herbicides. Moreover, it is possible that the distantly-related plant and microbial enzymes have distinct characteristics that could be exploited in the development of potent organism-specific inhibitors. Thus, a detailed understanding of the structure, mechanism, kinetics, and substrate-binding properties of the riboflavin biosynthetic enzyme(s), from plants for example, would serve as a starting point for the rational design of chemical compounds that might be useful as herbicides. Having the authentic plant protein(s) in hand would also provide a valuable tool for the in vitro screening of chemical libraries in search of riboflavin biosynthesis inhibitors.
Bacterial and fungal riboflavin biosynthesis has been intensively studied for more than four decades (For recent reviews, see Bacher, A., Chemistry and Biochemistry of Flavoproteins (Mxc3xcller, F., ed.) vol. 1, pp. 215-259 and 293-316 Chemical Rubber Co., Boca Raton, Fla. (1990)). The synthetic pathway consists of seven distinct enzyme catalyzed reactions, with guanosine 5xe2x80x2-triphosphate (GTP) and ribulose 5-phosphate the ultimate precursors. While the second and third steps of riboflavin biosynthesis occur in opposite order in bacteria and fungi, the remaining pathway intermediates are identical in both microorganisms. Structurally and mechanistically, the last two reactions in the pathway, namely, those catalyzed by 6,7-dimethyl-8-ribityllumazine synthase (LS) and riboflavin synthase (RS), are best characterized. In B. subtilis, these two enzymes are physically associated with each other in a huge spherical particle with a combined molecular mass of about 1 MDa (Bacher et al., J. Biol Chem. 255:632-637 (1980); Ritsert et al., J. Mol. Biol. 253, 151-167 (1995); Bacher et al., Biochem. Soc. Trans. 24(1):89-94 (1996)); the X-ray structure of the bifunctional protein complex has been determined at 3.3 angstrom resolution (Ladenstein et al., J. Mol. Biol 203:1045-1070). The LS/RS complex consists of 60 LS subunits that are organized into 12 pentamers to form a hollow icosahedral capsid. Encaged in the central core of this structure resides a single molecule of RS, a trimer of three identical subunits. Kinetic studies reveal that the compartmentation of the two enzymes within the complex improves the overall catalytic efficiency of riboflavin production at low substrate concentrations, presumably via xe2x80x9csubstrate channelingxe2x80x9d (Kis et al., J. Biol. Chem. 270:16788-16795 (1995)). Although a bifunctional LS/RS complex has not been observed in other microorganisms, it was recently shown that the native E. coli LS also exists in vivo as a hollow icosahedral capsid of 60 identical subunits (Mxc3x6rtl et al., J. Biol. Chem. 271:33201-33207 (1996)).
LS, the penultimate enzyme of riboflavin biosynthesis, catalyzes the condensation of 3,4-dihydroxy-2-butanone 4-phosphate with 4-ribitylamino-5-amino-2,6-dihydroxypyrimidine (RAADP) to yield 1 mol each of orthophosphate and 6,7-dimethyl-8-(1xe2x80x2-D-ribityl)-lumazine (DMRL). The latter is the immediate precursor of riboflavin. LS-encoding genes have been cloned from numerous microorganisms, including E. coli (Taura et al., Mol. Gen. Genet. 234:429-432 (1992)), A. pleuropneumoniae (Fuller et al., J. Bacteriol. 177:7265-7270 (1995)), P. phosphoreum (Lee et al., J. Bacteriol. 176:2100-2104 (1994)), B. subtilis (Mironov et al., Dokl. Akad. Nauk SSSR 305:482-487 (1989)), and S. cerevisiae (Garcia-Ramirez et al., J. Biol. Chem. 270:23801-23807 (1995)). In all cases, the subunit molecular mass of the LS gene product is small, ranging in size from xcx9c16-17 kDa.
While the various LS homologs all share certain structural features in common, their overall homology at the primary amino acid sequence level is rather poor. For example, as determined with the Genetics Computer Group Gap program (Wisconsin Package Version 9.0, Genetics Computer Group (GCG), Madison, Wis.), the E. coli LS is only 58%, 65%, 53%, and 36% identical to the homologous proteins of A. pleuropneumoniae, P. phosphoreum, B. subtilis and S. cerevisiae, respectively. Indeed, pairwise comparisons of these five proteins reveal that the two most similar homologs share only 72% identity.
The terminal step of riboflavin biosynthesis is mediated by RS. This enzyme catalyzes the dismutation of two molecules of DMRL to yield 1 mol of riboflavin and RAADP. That the latter product is also one of the substrates of LS explains in part the enhanced catalytic efficiency of the B. subtilis LS/RS complex noted above. Although the crystal structure of RS remains to be determined, it is surmised that the native bacterial (Bacher et al., J. Biol Chem. 255:632-637 (1980)) and fungal (Santos et al., J. Biol. Chem. 270:437-444 (1995)) proteins are trimers, each consisting of three identical xcx9c25 kDa subunits. To date, RS has only been cloned from about a dozen microorganisms, and all of the species that have been examined exhibit marked internal homology in their N-terminal and C-terminal domains (Schott et al., J. Biol. Chem. 265:4204-4209 (1990); Santos et al., J. Biol. Chem. 270:437-444 (1995)). Based on these observations, it has been suggested that the two halves of the RS protomer have arisen through gene duplication, and that each contains a substrate-binding site for DMRL.
Despite this structural similarity, however, the overall sequence homology of the various RS proteins is extremely limited. Thus, the E. coli RS protein is only 32%, 36%, 35%, and 31% identical to its counterparts in S. cerevisiae, P. phosphoreum., B. subtilis, and P. leiognathi; the GenBank accession numbers for the latter four proteins are Z21621, L11391, X51510 and M90094, respectively.
With the exception of GTP cyclohydrolase II, the first committed enzyme of riboflavin biosynthesis, virtually nothing is known about the riboflavin biosynthetic machinery of higher plants. The gene for this protein was recently cloned from an arabidopsis cDNA library (Kobayashi et al., Gene 160:303-304 (1995)). The protein sequence of the cloned plant gene is only 37-58% identical to the homologous proteins from E. coli, B. subtilis, P. leiognathi, and P. phosporeum. While full-length cDNA sequences have not been reported for any other plant riboflavin biosynthetic enzyme, the GenBank database contains two ESTs (Expressed Sequence Tags) that potentially correspond to plant LS genes. One of these is from castor bean and the other is from arabidopsis.
The castor bean cDNA clone (GenBank accession number T15152; van de Loo et al., Plant Physiol. 108:1141-1150 (1995)) is truncated at its 5xe2x80x2 end, and is missing DNA corresponding to at least 60 N-terminal amino acid residues. The arabidopsis cDNA clone (GenBank accession number Z34233; direct submission) was identified through a BLAST (Basic Local Alignment Search Tool; Altschul et al., J. Mol. Biol. 215:403-410 (1990)) search using the TBLASTN algorithm provided by the National Center for Biotechnology Information (NCBI). The query sequence for the BLAST search was the translated E. coli LS gene (GenBank accession number X64395) and the probability score for similarity to the arabidopsis EST was P=0.45. Unfortunately, the portion of the cDNA insert that was sequenced contained only the last 26 C-terminal amino acid residues of the protein, so it is not known whether it is a partial or full-length cDNA clone. Since neither of these clones possess a polyA tail, it is possible that they reflect contaminating microbial DNA that was introduced at some point during the preparation of the cDNA libraries.
In contrast to LS, BLAST searches failed to identify any plant DNA sequences in the GenBank database with significant primary amino acid sequence homology to either E. coli or yeast RS. However, RS activity has been detected in extracts from various plant species (Plaut, G., Metabolic Pathways (Greenberg, D. M., ed.), vol II, p. 673, Academic Press, New York, (1961)), and partial purification of the spinach homolog has been described (Mitsuda et al., Methods Enzymol. 18b:539-543 (1970)).
From the foregoing discussion, it is apparent that too little is known about plant LS or RS genes/proteins and their relationship to known microbial homologs to allow isolation of LS- or RS-encoding genes from any plant species using most classical approaches. The latter include hybridization probing of cDNA libraries with homologous or heterologous genes, PCR-amplification of the gene of interest using oligionucleotide primers corresponding to conserved amino acid sequence motifs, and/or immunological detection of expressed cDNA inserts in microbial hosts. Unfortunately, these techniques would not be expected to be very useful for the isolation of plant LS or RS genes, since they all heavily rely on the presence of significant structural similarity (i.e., DNA or amino acid sequence) with known proteins and genes that have the same function. Given the observation that LS and RS proteins are both so poorly conserved, even amongst microorganisms, it is highly unlikely that the known microbial homologs would share significant structural similarities with their counterparts in higher plants.
An alternative approach that has been used to clone biosynthetic genes in other metabolic pathways from higher eucaryotes is through complementation of microbial mutants that are deficient in the enzyme activity of interest. Since this strategy relies only on the functional similarity between the protein encoded for by the disrupted host gene and the target gene of interest, it is ideally suited for cloning structurally dissimilar proteins that catalyze the same reaction. For functional complementation, a cDNA library is constructed in a vector that can direct the expression of the cDNA in the microbial host. The plasmid library is then introduced into the mutant microbe, and colonies are selected that are no longer phenotypically mutant. Indeed, the LS (Garcxc3xada-Ramxc3xadrez et al., J. Biol. Chem. 270:23801-23807 (1995)) and RS (Santos et al., J. Biol. Chem. 270:437-444 (1995) of yeast, and arabidopsis GTP cyclohydrolase II (Kobayashi et al, Gene 160:303-304 (1995)) were all cloned through functional complementation of microbial riboflavin auxotrophs. This strategy has also worked for isolating genes from higher eucaryotes that are involved in other metabolic pathways, including lysine biosynthesis (Frisch et al., Mol. Gen. Genet. 228:287-293 (1991)), purine biosynthesis (Aimi et al., J. Biol. Chem. 265:9011-9014 (1990)), and tryptophan biosynthesis (Niyogi et al., Plant Cell 5:1011-1027 (1993)), and has also been successfully employed in the isolation of various plant genes including glutamine synthetase (Snustad et al., Genetics 120:1111-1124 (1988)), pyrroline-5-carboxylate reductase (Delauney et al., Mol. Genet. 221:299-305 (1990)), dihydrodipicolinate synthase (Frisch et al., Mol. Gen. Genet. 228:287-293 (1991)), 3-isopropylmalate dehydrogenase (Ellerstrom et al., Plant Mol. Biol. 18:557-566 (1992)), and dihydroorotate dehydrogenase (Minet et al., Plant J. 2:417-422 (1992)).
Despite the obvious attractive features of cloning by functional complementation, there are several reasons why this approach might not work when applied to the higher plant LS and RS genes. First, the eucaryotic CDNA sequence might not be expressed at adequate levels in the mutant microbe for a variety of reasons, including differences in preferred codon usage. Second, the cloned eucaryotic gene might not produce a functional polypeptide, if for instance, enzyme activity requires a post-translational modification, such as acetylation, glycosylation, or phosphorylation that is not carried out by the microbial host. Third, the heterologous plant protein might be lethal to the host, thus rendering its expression impossible. Fourth, the eucaryotic protein might fail to achieve its native conformation in the foreign microbial environment, due to folding problems, inclusion body formation, or various other reasons. It is also possible that the higher plant LS and RS enzymes are nuclear-encoded proteins that are posttranslationally targeted to chloroplasts, mitochondrial, or some other organelle that is not present in the microbial host. If this were the case and proteolytic removal of the organellar targeting sequence was required for enzyme activity, cloning these genes by functional complementation would not be possible.
The instant invention relates to isolated nucleic acid fragments encoding plant or fungal enzymes involved in riboflavin biosynthesis. Specifically, this invention concerns isolated nucleic acid fragments encoding a plant or fungal LS, wherein the plant is spinach, tobacco or arabidopsis and the fungus is Magnaporthe grisea. This invention also concerns isolated nucleic acid fragments encoding a plant or fungal RS, wherein the plant is spinach or arabidopsis and the fungus is Magnaporthe grisea. In addition, this invention relates to nucleic acid fragments that are complementary to nucleic acid fragments encoding a plant or fugal LS enzyme or a plant or fungal RS enzyme.
Specific isolated nucleic acid fragments encoding a plant LS enzyme are (a) an isolated nucleic acid fragment encoding all or a substantial portion of the amino acid sequence selected from the group consisting of SEQ ID NO:2, SEQ ID NO:4 and SEQ ID NO:6; (b) an isolated nucleic acid fragment that is substantially similar to an isolated nucleic acid fragment encoding all or a substantial portion of the amino acid sequence selected from the group consisting of SEQ ID NO:2, SEQ ID NO:4 and SEQ ID NO:6; (c) an isolated nucleic acid fragment encoding a polypeptide having at least 72% identity with the amino acid sequence selected from the group consisting of SEQ ID NO:2, SEQ ID NO:4 and SEQ ID NO:6; and (d) an isolated nucleic acid fragment that is complementary to (a), (b) or (c).
Special isolated nucleic acid fragments encoding a plant RS enzyme are (a) an isolated nucleic acid fragment encoding all or a substantial portion of the amino acid sequence selected from the group consisting of SEQ ID NO:8 and SEQ ID NO:10; (b) an isolated nucleic acid fragment that is substantially similar to an isolated nucleic acid fragment encoding all or a substantial portion of the amino acid sequence selected from the group consisting of SEQ ID NO:8 and SEQ ID NO:10; (c) an isolated nucleic acid fragment encoding a polypeptide having at least 70% identity with the amino acid sequence selected from the group consisting of SEQ ID NO:8 and SEQ ID NO:10; and (d) an isolated nucleic acid fragment that is complementary to (a), (b) or (c).
Specific isolated nucleic acid fragments encoding a fungal RS enzyme are (a) an isolated nucleic acid fragment encoding all or a substantial portion of the amino acid sequence set forth in SEQ ID NO:12; (b) an isolated nucleic acid fragment that is substantially similar to an isolated nucleic acid fragment encoding all or is homologous to at least a substantial portion of the amino acid sequence set forth in SEQ ID NO:12; (c) an isolated nucleic acid fragment that is complementary to (a) or (b).
Specific isolated nude acid fragments encoding a fungal LS enzyme are (a) an isolated nucleic acid fragment encoding all or a substantial portion of the amino acid sequence set forth in SEQ ID NO:38; (b) an isolated nucleic acid fragment that is substantially similar to an isolated nucleic acid fragment encoding all or is homologous to at least a substantial portion of the amino acid sequence set forth in SEQ ID NO:38; and (c) an isolated nucleic acid fragment that is complementary to (a) or (b).
In another embodiment, the instant invention relates to chimeric genes encoding a plant or fungal LS enzyme or a plant or fungal RS enzyme or to chimeric genes that comprise nucleic acid fragments that are complementary to the nucleic acid fragments encoding the enzymes, operably linked to suitable regulatory sequences, wherein expression of the chimeric genes results in production of levels of the encoded enzymes in transformed host cells that are altered (i.e., increased or decreased) from the levels produced in the untransformed host cells.
In a further embodiment, the instant invention concerns a transformed host cell comprising in its genome a chimeric gene encoding a plant or fungal LS enzyme or a plant or fungal RS enzyme, operably linked to suitable regulatory sequences, wherein expression of the chimeric gene results in production of altered levels of a plant LS enzyme or a plant or fungal RS enzyme in the transformed host cell. The transformed host cells can be of eucaryotic or procaryotic origin, and include cells derived from higher plants and microorganisms. The invention also includes transformed plants that arise from transformed host cells of higher plants, and from seeds derived from such transformed plants.
An additional embodiment of the instant invention, concerns a method of altering the level of expression of a plant or fungal LS enzyme or a plant or fungal RS enzyme in a transformed host cell comprising: a) transforming a host cell with the chimeric gene encoding a plant or fungal LS enzyme or a plant or fungal RS enzyme, operably linked to suitable regulatory sequences; and b) growing the transformed host cell under conditions that are suitable for expression of the chimeric gene wherein expression of the chimeric gene results in production of altered levels of LS or RS in the transformed cell.
An additional embodiment of the instant invention concerns a method for obtaining a nucleic acid fragment encoding all or substantially all of an amino acid sequence encoding a plant or fungal LS enzyme or a plant or fungal RS enzyme.
Additionally, an in vivo method is provided for identifying as an herbicidal or fungicidal candidate a chemical compound that inhibits the activity of a plant or fungal LS enzyme or a plant or fungal RS enzyme and thus serve as a crop protection chemical comprising the steps of: (a) disrupting the endogenous LS or RS gene of a suitable microbial host, rendering growth of the microbial host cell dependent on added riboflavin; (b) transforming the altered microbial host cell of step (a) with a chimeric gene comprising an isolated nucleic acid fragment encoding a plant or fungal LS enzyme or a plant or fungal RS enzyme, the chimeric gene operably linked to at least one suitable regulatory sequence that allows its expression in the microbial host cell; (c) growing the transformed host cell of step (a) under conditions suitable for expression of the chimeric gene plant or fungal LS or RS gene; (d) contacting the transformed microbial host cell with a chemical compound of interest in a well-controlled experiment while the host cell is growing exponentially and in both the presence and absence of added riboflavin; (e) identifying as an herbicide or fungicide candidate the chemical compound of interest that inhibits growth of the transformed microbial host cell only when grown in the absence of added riboflavin. Suitable isolated nucleic acid fragments are those set out above. Suitable microbial hosts for this in vivo assay include the E. coli LS and RS riboflavin auxotrophs that are described below, both of which normally require riboflavin supplementation for growth. Specific inhibition of the functional plant or fungal genes that are introduced into these mutants could then be assessed directly in parallel assays in which the transformed host cells are grown in both the presence and absence of added riboflavin. Those inhibitory compounds that only affect metabolic activity (growth) in the absence of riboflavin supplementation represent potential herbicides and/or fungicides.
In an alternate embodiment, an in vitro method is provided for identifying as an herbicide or fungicide candidate a chemical compound that inhibits the activity of a plant or fungal LS enzyme or a plant or fungal RS enzyme and thus serve as a crop protection chemical comprising the steps of: (a) transforming a host cell with a chimeric gene comprising a nucleic acid fragment encoding a plant or fungal LS enzyme or a plant or fungal RS enzyme, the chimeric gene operably linked to at least one suitable regulatory sequence; (b) growing the transformed host cell of step (a) under conditions suitable for expression of the chimeric gene resulting in the production of the plant or fungal LS enzyme or a plant or fungal RS enzyme; (c) purifying the plant or fungal LS enzyme or the plant or fungal RS enzyme expressed by the transformed host cell; (d) contacting the enzyme with a chemical compound of interest; and (e) identifying as an herbicide or fungicide candidate the chemical compound of interest that reduces the activity of the plant or fungal LS enzyme or plant or fungal RS enzyme relative to the activity of the respective enzyme in the absence of the chemical compound of interest. Such reduced activity indicates that the chemical compound is potentially useful as a crop protection chemical. Suitable isolated nucleic acid fragments are those set out above.